Metallocenes and other single-site complexes provide polyolefins with interesting physical properties that can be tailored to meet the needs of a particular application. They complement traditional Ziegler-Natta catalysts, which have multiple active sites and provide polymers with different attributes.
Metallocenes contain two cyclopentadienyl (“Cp”) or Cp-like ligands that coordinate through π-bonding to a transition metal, and the ligands are frequently linked via a divalent bridging group such as dialkylsilyl, methylene, or the like. Metallocenes in which the bridging group contains a siloxy (—Si—O—) moiety have been described, at least generally, although ways to prepare them are not disclosed. See, e.g., U.S. Pat. Nos. 7,109,278 and 5,278,264.
Other varieties of metallocenes incorporate a disiloxane bridge, in which an oxygen bridges two silacyclopentadienyl groups (see U.S. Pat. No. 7,060,765 and J. Polym. Sci. A 42 (2004) 3323). The disiloxy-bridged complexes are typically made by reacting indenyllithium with dichlorotetramethyldisiloxane; they cannot be made directly from indanone enolates. Metallocenes that include siloxy substituents that are not part of the bridging group are also known (see U.S. Pat. Nos. 7,074,863; 7,037,872; and 6,316,556; and Organometallics 15 (1996)2450). In other non-metallocene complexes, the siloxy bridge is part of a “constrained geometry” complex (see U.S. Pat. No. 7,074,863 at col. 13) or is joined to a non-Cp group (see U.S. Pat. No. 6,872,843). In the '843 patent, the siloxy-bridged complex is made by reacting an indenylchlorodimethylsilane with an aldimine-functionalized phenoxide salt derived from salicylaldehyde.
References that generally disclose siloxy-bridged metallocenes (such as the '278 and '264 patents noted earlier) do not teach how to make them. Interestingly enough, these same references illustrate that indanones are commonly converted to indenes in the early stages of making bridged ligand precursors and transition metal complexes that incorporate the precursors. (See, for example, the '278 patent at Examples 1e, 2d, and 3e; and the '264 patent at Examples I.3; II.2; and III.3; see also U.S. Pat. No. 7,074,863 at cols. 8-28.) Thus, the indanone is usually converted to an alcohol by reduction or Grignard addition, followed by acid-catalyzed dehydration to provide an indene. The indene is then further elaborated to make the bridged ligand precursor.
Preparation of Bridged Metallocene Complexes from Indenes, Particularly substituted indenes, is further complicated by the possibility of making multiple regioisomers. Consider, for instance, the simple case of 2,4-dimethylindene, which provides two different stereoisomers in the following reaction:
Because deprotonation gives two different indenides, halide displacement can occur at either of two carbons. Such stereoisomers are not usually easy to separate, so the catalysts ultimately made will be mixtures of different compounds; this defeats the purpose of having a “single-site” catalyst. Conversely, substituents on the indene are often considered valuable for influencing catalyst activity or polymer properties. Thus, indenes are not optimum starting materials for making metallocenes.
In sum, there is a continuing need for efficient ways to make bridged metallocene complexes. A valuable approach would be simple to practice with readily available reagents and would avoid the multistep approach now used commonly to make indenyl complexes. Ideally, the method would utilize readily available indanones as starting materials, including substituted indanones, and would enable an efficient synthesis of bridged ligand precursors and transition metal complexes while maintaining control over regiochemical outcome.